On sailing of a ship, a dangerous place exists where a collision accident or accident avoidance behavior is found more frequently than another area. Accordingly, a technique exists for executing risk determination for sailing of ships based on a relative distance between the ships.
Japanese Laid-open Patent Publication No. 2014-178258
Japanese Laid-open Patent Publication No. 10-250681
However, even though risk determination for sailing of ships is executed based on a relative distance between the ships, it may be impossible to detect a dangerous place with a high risk of collision accurately. For example, sailing in a harbor includes both entry into a harbor facility and continued sailing on a sailing route in a harbor. Accordingly, in risk determination based on a relative distance between ships, a course change for entry into a wharf as a destination from a sailing route may be erroneously detected as avoidance behavior.